Effect of temporal variability in infiltration on contaminant transport in the unsaturated zone

A general methodology has been developed for investigating the effect of short-term temporal variability in infiltration on the long-term transport of contaminants in soils. A one-dimensional, unsaturated transport model was used to simulate the transport of a sorbing, nonvolatile solute, using either steady-state or randomly varying infiltration. Concentration breakthrough curves are plotted against time and cumulative infiltration for constant rainfall, and for five, random-rainfall realizations. The observed time-based breakthrough curves for an individual year depend significantly on the actual rainfall pattern for that year. The average breakthrough curve, generated from many years of randomly generated rainfall, approaches the constant infiltration time breakthrough curve. For an individual year, the cumulative infiltration breakthrough curves for variable and constant infiltration match closely, as suggested by Wierenga, P.J. [Wierenga, P.J., 1977. Solute distribution profiles computed with steady-state and transient water movement models. Soil Sci. Soc. Am. J., 41, 1050]. This indicates that, for the conditions examined, cumulative rainfall can be used to predict adequately contaminant transport for a given time period. Under more severe conditions, increased variability in infiltration is expected to increase dispersion. A dimensional analysis of the governing equations indicates two additional explanations for the influence of infiltration variability on contaminant transport. First, the hydraulic conductivity (and therefore, the velocity) and retardation factor depend on the soil water content, which depends on the infiltration pattern. Second, molecular diffusion dominates transport during dry periods. The impact of this diffusion on the overall contaminant transport depends on the duration of dry periods.